A fuel cell system is an energy conversion system in which a fuel gas and an oxide gas are supplied to a film-electrode bonded member to cause an electrochemical reaction, and chemical energy is converted into electric energy. Above all, in a solid polymer electrolytic fuel cell in which a solid polymer film is used as an electrolyte, compactness can easily be achieved at low cost, further the cell has a high output density, and hence an application as a power supply system to be mounted on a vehicle is expected.
Furthermore, in a fuel cell vehicle on which the fuel cell system is mounted as a power source, in a high output area where power generation efficiency is high, a fuel cell stack is allowed to generate power, and the power is supplied to a traction motor from both or one of the fuel cell stack and a secondary cell, whereas in a low output area where the power generation efficiency is poor, the power generation of the fuel cell stack is temporarily halted, and operation control is performed so as to supply the power only from the secondary cell to the traction motor. An operation method in which the power generation of the fuel cell stack and the halting of the power generation are alternately repeated in accordance with a size of a load of the fuel cell stack in this manner is referred to as an intermittent operation. In a low load area where the power generation efficiency of the fuel cell stack is low, the intermittent operation to temporarily halt the operation of the fuel cell stack is performed, so that the fuel cell stack can be operated in a region in which energy conversion efficiency is high, and the efficiency of the whole fuel cell system can be improved.
From a viewpoint of a power generation characteristic of the fuel cell stack, even when control shifts from a power generation halted state to a power generation state, a considerably long time is required for the supply of the fuel gas and the oxide gas to the fuel cell stack, progress of the electrochemical reaction in the film-electrode bonded member, the generation of the power corresponding to a required power and the like. Therefore, the required power cannot instantly be output, and delay in the power generation is sometimes caused. In the vehicle on which the fuel cell system is mounted as the power source to be mounted on the vehicle, such delay in the power generation becomes a factor for deterioration of drivability.
To solve such a problem, in Japanese Patent Application Laid-Open No. 2004-172028, a method is suggested in which at a time when a cell voltage of the fuel cell stack which has performed the intermittent operation to thereby shift to the power generation halted state becomes below a predetermined value, an air compressor is driven to supply the oxide gas to the fuel cell stack, oxygen shortage in a cathode of the fuel cell stack is solved to recover the cell voltage, and response delay with respect to the request for the power generation is suppressed.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-172028